1. Field of the Invention
The invention relates to the class of test devices known as reflectometers which are used for measuring the reflectivity of mirrors or other reflecting devices.
2. Description of the Prior Art
Developments in the fields of laser gyroscopes and high energy beam steering have required the production of mirrors having exceptionally high reflectivities. In parallel with the requirement for attaining high reflectivities, there exists the requirement to measure the reflectivity of such mirrors to extremely high precision. For many present day systems, mirror reflectivities R in excess of 0.99 are required. Unfortunately, most prior art reflectivity measuring techniques permitted the reflectivity to be measured to an accuracy of no greater than 1%. Hence, the error in the measurement of the loss L, defined as 1-R, of such high reflectivity mirrors could be more than 100%.
In the past, the most common technique for measuring the reflectivity of the mirror was to determine the proportions of energy incident upon a mirror which were lost due to transmission, absorption, and scattering, the reflectivity then being computed by simply subtracting the sum of these from unity. Typically, the fraction S scattered would be 0.002, the fraction A absorbed 0.002, and the fraction T transmitted 0.0003. T may be determined most simply by measuring the incident and the transmitted intensities for given beam of light. This measurement can be done to an accuracy of approximately 1%. S, the fraction scattered, may be measured to a somewhat lesser accuracy by the use of an integrating sphere. A, the fraction absorbed, is the most difficult of the three quantities to be measured and can be determined to an accuracy of only about 10% with the use of calorimetry techniques if a sufficiently powerful light source is available. For the wavelengths typically used in laser gyroscopes and in the visible spectrum, a sufficiently powerful source is not generally available which would permit accurate determination of A. In summary, this technique is both cumbersome and inaccurate for the types of measurement for which the present invention is intended.